This invention relates to receivers for decoding signals received via multiple propagation paths having different propagation delays and, more particularly, to use of filters periodically adapted to center frequencies of variations of numerical estimates of amplitude and phases of the delayed propagation paths.
Radio receivers are often used for decoding fading signals with the aid of estimates of the instantaneous propagation channel phase and amplitude characteristics. An exemplary application for such a radio receiver is a cellular phone for decoding signals transmitted by base stations using code division multiple access (CDMA) protocols.
A radio receiver, such as for a CDMA system, receives digitally coded and modulated signals from a transmitter. These signals include known, preselect signal patterns at known time intervals. Using known signal patterns and, optionally, data signals obtained after data decoding, the receiver forms successive estimates of the phase and amplitude or complex value of propagation path characteristics between the transmitter and the receiver. These include estimates for multiple paths in the case of multi-path propagation.
It is desirable to smooth the sequence of successive channel estimates to reduce noise and estimation error. A smoothing filter that has a symmetrical filter response is appropriate when the fading spectrum is symmetrical about zero frequency, as expected with a long term average. However, in the short term, on the order of seconds, the fading spectrum may be asymmetrical due to non-uniform distribution of the angle of arrival of multi-path arrays.
The use of channel estimation from a received radio signal using both known symbols embedded in the signal, as well as unknown information symbols that are decoded by the receiver, are well known in the art. Examples of such receivers are shown in U.S. Pat. Nos. 5,331,666; 5,335,250; 5,557,645; and 5,619,533, and also U.S. patent application Ser. No. 08/305,727, filed Sep. 14, 1994, all of which are incorporated by reference herein. Exemplary receivers using channel estimation specific to CDMA systems are shown in U.S. Pat. Nos. 5,151,919 and 5,218,619, also incorporate by reference herein.
Smoothing of channel estimates may be accomplished using a Finite Impulse Response (FIR) filter having a series of complex coefficients. Discussion on smoothing channel estimates using FIR filter, or autoregression, may be found in xe2x80x9cAdaptive Equalization For Mobile Radio Channelsxe2x80x9d (Licentiate Thesis, Lars Lindblom, Uppsala University 1992, ISSN 03468887), which is also incorporated by reference herein. This paper discusses the benefit of adapting a smoothing filter""s characteristics to the fading spectrum of the signal. However, in the prior art the fading spectrum of a signal was assumed to be symmetrical. Over the long term, for example several minutes, the fading spectrum may be symmetrical in accordance with Jake""s model for fading in the urban, mobile radio propagation environment. The use of Jake""s model and modifications thereof to speed computation during simulations of communications system performance may be found in a paper entitled xe2x80x9cJake""s Fading Model Revisitedxe2x80x9d by Dent et al., ELECTRONICSLETTERS, Jun. 24, 1993, Volume 29, No. 13, page 1162 et seq., which paper is incorporated by reference herein.
Jake""s model assumes a uniform angular distribution of reflecting objects around a mobile receiver. The relative Doppler shift of reflected signals arising from different angles relative to the direction of movement varies with the cosine of the angle of arrival. With a uniform angular distribution, the Doppler spectrum is then symmetrical and two sided, having as much reflected energy arriving from behind the mobile receiver with a negative Doppler frequency shift as from ahead of the receiver, having a positive Doppler frequency shift. Rays reaching the receiver from behind have clearly not propagated an equal distance from transmitter to receiver as rays reaching the receiver from the front. However, these delay differences were ignored in the prior art. Jake""s model assumed that rays with such delay differences could nevertheless be combined to produce a net fading waveform for a path of delay equal to the mean of these rays. More specifically, delays lying within a plus or minus 0.5 of a modulation symbol period of each other were combined to produce a net fading ray with a mean delay. Delays outside that plus or minus 0.5 modulation symbol period were grouped into a different plus or minus 0.5 symbol window to obtain a different net fading waveform with a different mean delay. The different net fading waveforms with their associated modulation-symbol-space delays were then taken to characterize a multi-path channel. Each of the multiple paths is nevertheless assumed to conform to Jake""s fading model, i.e., each path is the combination of rays arriving uniformly from all directions.
In a wide band CDMA system (WBCDMA), modulation symbol intervals are much shorter. This allows multiple propagation paths to be resolved with much finer time resolution. Thus, it is no longer valid to use a Jake""s model which adds rays that differ in their propagation delay by even a fraction of a microsecond. This addition was valid only in the context of narrow band FDMA or medium bandwidth TDMA systems. In WBCDMA systems, it is necessary to restrict combination of different rays reaching the receiver to rays that have the same propagation delay from the base station to the mobile station, within plus or minus 0.5 of a CDMA chip duration. In a five MHZ wide WBCDMA system, a chip duration is typically 0.25 microseconds so that plus or minus 0.5 chips is plus or minus 0.125 microseconds, or plus or minus 37.5 meters expressed as a propagation path length variation. It may be shown that rays with the same delay to this order of accuracy must have reflected from objects lying on an elliptical contour having the base station and the mobile station as its foci. These objects are not any longer uniformly angularly spaced around the mobile receiver, nor are they spaced at the same distance from either the mobile station or the base station. Moreover, since the base station lies inside the elliptical contour, if, as is usual, it employs directional transmit antennae, objects around the elliptical contour will not be uniformly illuminated. Consequently, the fading spectrum of a ray of given delay within plus or minus 0.5 chip periods are no longer symmetrical about zero frequency. In addition, the offset from zero frequency of the centroid of the fading spectrum is no longer independent of the direction of motion. Consequently, the assumptions of the prior art used in channel estimation and smoothing of channel estimates are overly pessimistic as regards to the bandwidth of the fading.
A claim made in the published art for WBCDMA signals is that the high time resolution enables resolution of individual reflecting objects such that each resolved ray is a single, non-fading ray, i.e., WBCDMA xe2x80x9celiminates fadingxe2x80x9d. It is recognized that such xe2x80x9cnon-fadingxe2x80x9d rays will come and go, but on the relatively longer time scale of log normal shadowing, which is easier to track. However, each ray has a varying Doppler frequency, which means that its phase still varies at up to the Doppler rate, even if its amplitude varies much slower. Thus, there remains the need to track the varying complex value of the propagation channel in order to effect coherent signal decoding, i.e., with knowledge of a phase reference. Moreover, the complete elimination of fading by resolving small reflecting objects is not achieved except using very large bandwidths, beyond the bandwidths of anticipated WBCDMA systems, which therefore find themselves in the intermediate region of propagation paths that still each comprise multiple rays. Fading models and channel estimation means for these WBCDMA have not been addressed in the prior art.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.
In accordance with the invention a receiver uses a smoothing filter having an asymmetrical frequency response and adapted to the short term spectral shape of each of resolvable multi-path fading waveforms.
Broadly, there is disclosed herein a receiver for decoding signals received via multiple propagation paths having different propagation delays. The receiver includes receive means for receiving the signals and converting the signals to digital samples for processing. First processing means are operatively associated with the receive means for correlating periodically selected groups of the digital samples for responding to known symbols and periodically producing numerical estimates related to amplitudes and phases of the delayed propagation paths. Filter means filter the numerical estimates using filters periodically adapted to center frequencies of variations of the numerical estimates. Second processing means are operatively associated with the receive means and the filter means for decoding data using the filtered estimates and the digital data.
It is a feature of the invention that the receive means is adapted to receive code division multiple access (CDMA) signals.
It is another feature of the invention that the first processing means comprises a matched filter matched to the known symbols.
It is another feature of the invention that the known symbols comprise pilot symbols spread with a known CDMA spreading code. The pilot symbols recur in the signals every 0.625 milliseconds.
It is another feature of the invention that the filter means comprises a Finite Impulse Response (FIR) filter. The FIR filter processes a stream of real estimate values and a stream of corresponding imaginary estimate values jointly using a set of complex coefficients. The FIR filter comprises a minimum phase filter.
It is a further feature of the invention that the filter means comprises a running average filter. The running average filter is compensated for frequency offsets in filtered values.
It is still another feature of the invention that the filter means has an asymmetrical frequency response.
It is yet another feature of the invention that the second processing means comprises a rake receiver.
It is an additional feature of the invention to provide third processing means operatively associated with the first processing means for estimating the center frequencies using the periodically produced numerical estimates and updating the filter means with the estimated center frequencies. The third processing means computes a complex autocorrelation. Particularly, the third processing means computes a complex Fourier transform.
There is disclosed in accordance with another aspect of the invention a receiving apparatus comprising a receiver receiving signals via multiple propagation paths having different propagation delays and converting the signals to digital samples for processing. A channel estimator is operatively coupled to the receiver for correlating periodically selected groups of the digital samples corresponding to known symbols and periodically producing numerical estimates related to amplitudes and phases of the delayed propagation paths. A filter system filters the numerical estimates using filters periodically adapted to center frequencies of variations of the numerical estimates. A data decoder is operatively coupled to the receiver and the filter system for decoding data using the filtered estimates and the digital data.
There is disclosed in accordance with a further aspect of the invention the method of decoding signals received via multiple propagation paths having different propagation delays, comprising the steps of receiving the signals and converting the signals to digital samples for processing, periodically correlating selected groups of the digital samples corresponding to known symbols and periodically producing numerical estimates related to amplitudes and phases of the delayed propagation paths, filtering the numerical estimates using filters periodically adapted to center frequencies of variations of the numerical estimates, and decoding data using the filtered estimates and the digital data.
More particularly, in one embodiment of the invention, the centroid of the fading spectrum is determined by processing successive channel estimates using any spectral estimation technique, such as Fourier transform. The frequency offset from zero frequency of the spectral centroid is then utilized to center the frequency response of a smoothing filter. The smoothing filter can then have a narrower bandwidth and suppress more noise then a prior art non-centered filter. Centering the filter includes applying a successive phase twist to the successive channel estimates to center the fading spectrum for filtering with a symmetrical filter. Alternatively, an FIR filter can have a series of complex coefficients which embody the successive phase twist to produce a frequency offset response.
In a preferred embodiment, the smoothing filter frequency response is matched to the asymmetrical fading spectrum of each path individually. Successive estimates for a propagation path are processed to determine a fading spectrum using, for example, a Fast Fourier Transform (FFT). The modulus of the spectral components is formed as an estimate of the square root of the power spectrum. The modulus spectrum, including asymmetries about zero frequency caused by frequency error and asymmetrical distribution of Doppler components, is used to determine a filter for smoothing the estimates of that propagation path""s characteristics in a future time period to reduce noise on the estimates. For example, the inverse FFT of the modulus spectrum can be used to determine complex coefficients of a complex FIR filter to be applied in the time domain. The complex coefficients exhibit systematic phase twists in correspondence to the modulus spectrum asymmetry. Propagation path estimates filtered using a matched filter structure having the calculated FIR coefficients are then used to improve data decoding using, for example, a rake receiver in the case of CDMA signals in which the filtered path estimates are used as tap weights. The FIR coefficients may be updated only at the rate at which the short term fading spectrum is expected to change. For example, sixteen channel estimates may be computed every ten milliseconds and processed by a sixteen-point FFT to obtain a new power spectrum. The new power spectrum may be combined with a previously averaged power spectrum to obtain a new average power spectrum using, for example, xe2x80x9cexponential forgettingxe2x80x9d in which earlier power spectra are given a lower weight in the average than more recent spectra.
Spectral components greater than a maximum expected Doppler frequency may be discarded. Alternatively, a noise floor may be estimated and components at or below the noise floor may be discarded. With another alternative, the location of most spectral energy can first be determined, and then an expanding box placed about that frequency centroid which is allowed to increased in width until it is deemed to encompass all spectral components of significance. After processing by any of the above techniques to delete insignificant spectral components, the square root of the remaining components are then computed and inverse Fourier transformed to determine a set of complex FIR coefficients for use as a smoothing filter for a path. In this way, each path""s fading channel estimates become filtered with a filter adapted to their particular fading characteristics, thus improving noise suppression.